A sliver of moon

February 8, 2011

A thin, crescent moon started appearing in the western sky over the weekend. The crescent will thicken as the moon's orbit carries it farther from the sun in our sky and shows us more of the sunward side. As it does, it will move along the Ecliptic through the constellations of the Zodiac and the winter hexagon.

The figure shows its positions next Saturday, Sunday, Monday and Tuesday when it will be in the waxing gibbous phase, between the first quarter and full moons. Its motion is a guide to the Ecliptic, the annual path of the sun. On Sunday, the moon will be very close to the summer solstice. The sun will be at that position on June 21, 2011 at 5:16 p.m., the first day of summer and the longest day of the year.

Notice how the moon moves from slightly above the Ecliptic to slightly below it. This is because the moon's orbit is inclined a little more than 5 degrees from Earth's. If it weren't for this tilt, there would be eclipses every month! The tilt causes the shadows of Earth and Moon to be above or below the other body most months. Eclipses occur only when the new or full moon is on or very close to the ecliptic. This month, it's the waxing gibbous moon on the ecliptic.

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The full moon was on the ecliptic on the day of the winter solstice in December, when there was a total lunar eclipse, inconveniently hidden from Adirondack observers by clouds! The full moon in June will also be on the Ecliptic so there will be another total lunar eclipse. Unfortunately, this one will be hidden from the Adirondacks by the Earth itself and be visible only from Asia, Europe and Africa.

The bright moon will obscure the dimmer stars of the winter constellations, but that can make it easier for beginning stargazers to learn the names of brighter stars. Astronomers quantify the brightness of stars as apparent magnitudes. This was begun by Hipparchus, who is believed to have made his observations from the Greek island of Rhodes between 147 and 127 B.C.E. ("Before the Common Era" is the new name for B.C. and Common Era is the new name for A.D.).

In his star catalog later used by Ptolemy, Hipparchus ranked the brightest stars as magnitude 1, the next brightest as magnitude 2 and on through magnitude 6 for the dimmest stars he could see. This system makes perfect sense until one realizes that it means dimmer objects have higher numbers. The barely discernable blur of the galaxy in Andromeda (discussed in The Wilderness Above on Nov. 20, 2010) has an apparent magnitude of 3.4, whereas the moon's is minus 12.92 and the sun's is minus 26.74. The system has gone through some adjustments since Hipparchus, so Sirius, at magnitude of minus 1.47, is the brightest star other than the sun. Capella is the sixth brightest star in the sky and one of a number of "zeroth magnitude" stars at 0.06. The others in the winter hexagon are Rigel (RYE'-JEL , 0.15), Procyon (PRO'-SEE-ON, 0.37), Betelgeuse (BET'-EL-JOOZ, 0.43), and Aldebaran (AL-DEB'-UH-RAHN, 0.84). Most of the rest of the stars named on the figure are first magnitude stars. Saiph (SAFE), Mintaka (MIN-TA'-KA), Aludra (AL-UDE'-RAH) and Hassaleh (HA-SAL'-LAY) are second magnitude and Meissa (MAY'-SAH) in Orion the only third magnitude one labeled. At 3.37, its magnitude is almost the same as the Andromeda galaxy, but being a star instead of a diffuse blur, it's much easier to see.

When I point out the constellations of the winter hexagon to my students, I often have one familiar with horoscopes ask, "Isn't Gemini a summer constellation?" I point out that a person's "sun sign" is the constellation the sun is in at the time of the person's birth (the astrological constellation that differs somewhat from the astronomical constellation as was discussed in all the media a couple weeks ago).

When the sun is "in" a constellation as it is in Capricornus today, that constellation is invisible due to the glare of the sun. In the summer, the sun is in Gemini, so it is not visible to us at night. Horoscopic astrology was developed during the first millennium B.C.E. Thus the Babylonian astronomers upon whose star maps and catalogs it was based, observed the skies for enough years and with enough accuracy to know which stars were behind the sun on any given day of the year! As we learn the names of the stars, we call many by the same names used by those skilled, ancient astronomers, see skies virtually unchanged since they did their careful work and share with them our appreciation of the beauty and fascination with our incredible universe.

If you have any questions about astronomy, please visit the Adirondack Public Observatory web site at apobservatory.org or email Aileen at aodonoghue@stlawu.edu.